There has a been a trend in the field of disposable diaper manufacturing to improve leakage prevention while making the disposable diaper appear and fit more like the undergarments they are meant to replace. This goal is best achieved by a disposable diaper with curved leg elastics.
Curving the leg elastics has been very difficult to accomplish and practice at high speed. Manufacturers have settled for making disposable diapers having straight bands of elastic.
The application of an elastic in a steeply curved pattern to a moving web requires the use of rollers for positioning the strip of elastic. U.S. Pat. No. 2,592,581 to Lorig discloses a method and apparatus for positioning a strip utilizing a roll which may be cylindrical, concave, or convex as desired to suit various installations. The U.S. Pat. No. 4,081,301 to Buell and U.S. Pat. No. 4,227,952 to Sabee disclose typical arrangements for applying elastic along a straight line to a web. In the Buell patent, the elastic and web substrate are continuously run at high speed while adhesive is intermittently applied to the elastic material.
The U.S. Pat. No. 3,828,367 Bourgeois discloses an apparatus for applying curved elastic to a disposable garment In the Bourgeois patent, a pair of elastic ribbons are fed to curved grooves in a roll under which a continuous web passes. As the roll with the ribbons in its grooves goes over the web, the ribbons are transferred in the contoured pattern of the grooves to the roll. One problem with a grooved roll for applying elastic ribbon to a web is that a groove which is sufficiently deep to guide the ribbon is too deep to apply a flat elastic ribbon. A curved groove is unreliable insofar as preventing roping and C-folding of a flat ribbon.
The U.S. patent application Ser. No. 621,900, filed June 18, 1984 and assigned to the assignee of the present application discloses a method and apparatus for applying elastic ribbon to a web along undulated path. The method and apparatus involve a single roll moving or oscillating in a single dimension to apply each elastic ribbon.
The present invention provides an improved means for applying elastic in a moderately steep curved pattern to a moving web using an applicator roll which oscillates in a direction transverse to the direction of the web and whose axis is parallel to the surface thereof. Due to space limitations, a horizontally oriented roll is necessary in order to obviate twisting of the elastic ribbon. Further, the coefficient of friction between the elastic ribbon and the surface of the oscillating roll bears a relationship to the transverse distance over which the roll oscillates in order to apply elastic having a moderately sharp degree of curvature.